High temperature oxidation protection for composites

ABSTRACT

The present disclosure provides a method for coating a composite structure, comprising applying a single pretreating composition on a surface of the composite structure, the single pretreating composition comprising a first acid aluminum phosphate comprising a molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3, and heating the composite structure to a first temperature sufficient to form an aluminum phosphate polymer layer on the composite structure.

FIELD

The present disclosure relates generally to carbon-carbon composites and, more specifically, to oxidation protection systems for carbon-carbon composite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typically designed to minimize loss of carbon material due to oxidation at operating conditions, which include temperatures as high as 900° C. (1652° F.). Phosphate-based oxidation protection systems may reduce infiltration of oxygen and oxidation catalysts into the composite structure. However, despite the use of such oxidation protection systems, significant oxidation of the carbon-carbon composites may still occur during operation of components such as, for example, aircraft braking systems.

SUMMARY

A method for coating a composite structure is provided, comprising applying a single pretreating composition on a surface of the composite structure, the single pretreating composition comprising a first acid aluminum phosphate comprising a first molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3, and heating the composite structure to a first temperature sufficient to form an aluminum phosphate polymer layer on the composite structure. In various embodiments, the method may further comprise applying a second slurry to the aluminum phosphate polymer layer, and heating the composite structure to a second temperature sufficient to form a sealing layer on the aluminum phosphate polymer layer.

In various embodiments, the second slurry may comprise a second pre-slurry composition and a second carrier fluid, wherein the second pre-slurry composition comprises a second phosphate glass composition. In various embodiments, the method may further comprise applying a first slurry to the aluminum phosphate polymer layer and heating the composite structure to a third temperature sufficient to form a base layer prior to the applying the second slurry to the aluminum phosphate polymer layer, wherein the first slurry comprises a first pre-slurry composition and a first carrier fluid, wherein the first pre-slurry composition comprises a first phosphate glass composition. In various embodiments, the first pre-slurry composition may comprise a second acid aluminum phosphate wherein a second molar ratio of aluminum to phosphate is between 1 to 2 and 1 to 3. In various embodiments, the first molar ratio of the aluminum to phosphate in the first acid aluminum phosphate may be between 1 to 2 and 1 to 2.7.

In various embodiments, the method may further comprise applying a barrier coating to the composite structure prior to the applying the single pretreating composition to the composite structure. The barrier coating may comprise at least one of a carbide, a nitride, a boron nitride, a silicon carbide, a titanium carbide, a boron carbide, a silicon oxycarbide, a molybdenum disulfide, a tungsten disulfide, or a silicon nitride.

In various embodiments, at least one of the first phosphate glass composition or the second phosphate glass composition may be represented by the formula a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z):

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y₁+y₂+z)=1; and

x<(y₁+y₂).

In various embodiments, the first slurry may comprise a refractory compound such as a nitride, a boron nitride, a silicon carbide, a titanium carbide, a boron carbide, a silicon oxycarbide, silicon nitride, molybdenum disulfide, or tungsten disulfide. In various embodiments, at least one of the first slurry or the second slurry may comprise at least one of a surfactant, a flow modifier, a polymer, ammonium hydroxide, ammonium dihydrogen phosphate, acid aluminum phosphate, nanoplatelets, or graphene nanoplatelets.

In various embodiments, a single pretreating composition may comprise an acid aluminum phosphate, wherein the acid aluminum phosphate may comprise a molar ratio of aluminum to phosphate is between 1 to 2 and 1 to 3. In various embodiments, the single pretreating composition may be substantially free of aluminum oxide. In various embodiments, the molar ratio of aluminum to phosphate in the acid aluminum phosphate may be between 1 to 2 and 1 to 2.7. In various embodiments, the molar ratio of aluminum to phosphate in the acid aluminum phosphate may be 1 to 2.5.

In various embodiments, an article may comprise a carbon-carbon composite structure, an oxidation protection system including an aluminum phosphate polymer layer disposed on an outer surface of the carbon-carbon composite structure, the aluminum phosphate polymer layer having a first molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3. In various embodiments, the article may further comprise a sealing layer disposed on the aluminum phosphate polymer layer. The sealing layer may comprise a second phosphate glass composition. In various embodiments, the oxidation protection system may further comprise a base layer disposed between the aluminum phosphate polymer layer and the sealing layer, wherein the base layer may comprise a first phosphate glass composition and an acid aluminum phosphate. In various embodiments, the base layer may comprise a second molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed out and distinctly claimed in the concluding portion of the specification. A more complete understanding of the present disclosure, however, may best be obtained by referring to the detailed description and claims when considered in connection with the drawing figures, wherein like numerals denote like elements.

FIG. 1A illustrates a cross sectional view of an aircraft wheel braking assembly, in accordance with various embodiments;

FIG. 1B illustrates a partial side view of an aircraft wheel braking assembly, in accordance with various embodiments;

FIGS. 2A, 2B, and 2C illustrate methods for coating a composite structure, in accordance with various embodiments;

FIG. 3 illustrates experimental data obtained from testing various glass compositions, in accordance with various embodiments;

FIG. 4 illustrates further experimental data obtained from testing various glass compositions, in accordance with various embodiments at 760° C. (1400° F.);

FIG. 5 illustrates further experimental data obtained from testing various oxidation protection slurries, in accordance with various embodiments at 760° C. (1400° F.); and

FIG. 6 illustrates further experimental data obtained from testing various oxidation protection systems, in accordance with various embodiments at 760° C. (1400° F.).

DETAILED DESCRIPTION

The detailed description of embodiments herein makes reference to the accompanying drawings, which show embodiments by way of illustration. While these embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, it should be understood that other embodiments may be realized and that logical and mechanical changes may be made without departing from the spirit and scope of the disclosure. Thus, the detailed description herein is presented for purposes of illustration only and not for limitation. For example, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option.

With initial reference to FIGS. 1A and 1B, aircraft wheel braking assembly 10 such as may be found on an aircraft, in accordance with various embodiments is illustrated. Aircraft wheel braking assembly may, for example, comprise a bogie axle 2, a wheel 14 including a hub 16 and a wheel well 18, a web 20, a torque take-out assembly 22, one or more torque bars 24, a wheel rotational axis 26, a wheel well recess 28, an actuator 30, multiple brake rotors 32, multiple brake stators 34, a pressure plate 36, an end plate 38, a heat shield 40, multiple heat shield sections 42, multiple heat shield carriers 44, an air gap 46, multiple torque bar bolts 48, a torque bar pin 50, a wheel web hole 52, multiple heat shield fasteners 53, multiple rotor lugs 54, and multiple stator slots 56. FIG. 1B illustrates a portion of aircraft wheel braking assembly 10 as viewed into wheel well 18 and wheel well recess 28.

In various embodiments, the various components of aircraft wheel braking assembly 10 may be subjected to the application of compositions and methods for protecting the components from oxidation.

Brake disks (e.g., interleaved rotors 32 and stators 34) are disposed in wheel well recess 28 of wheel well 18. Rotors 32 are secured to torque bars 24 for rotation with wheel 14, while stators 34 are engaged with torque take-out assembly 22. At least one actuator 30 is operable to compress interleaved rotors 32 and stators 34 for stopping the aircraft. In this example, actuator 30 is shown as a hydraulically actuated piston, but many types of actuators are suitable, such as an electromechanical actuator. Pressure plate 36 and end plate 38 are disposed at opposite ends of the interleaved rotors 32 and stators 34. Rotors 32 and stators 34 can comprise any material suitable for friction disks, including ceramics or carbon materials, such as a carbon/carbon composite.

Through compression of interleaved rotors 32 and stators 34 between pressure plates 36 and end plate 38, the resulting frictional contact slows rotation of wheel 14. Torque take-out assembly 22 is secured to a stationary portion of the landing gear truck such as a bogie beam or other landing gear strut, such that torque take-out assembly 22 and stators 34 are prevented from rotating during braking of the aircraft.

Carbon-carbon composites (also referred to herein as composite structures, composite substrates, and carbon-carbon composite structures, interchangeably) in the friction disks may operate as a heat sink to absorb large amounts of kinetic energy converted to heat during slowing of the aircraft. Heat shield 40 may reflect thermal energy away from wheel well 18 and back toward rotors 32 and stators 34. With reference to FIG. 1A, a portion of wheel well 18 and torque bar 24 is removed to better illustrate heat shield 40 and heat shield segments 42. With reference to FIG. 1B, heat shield 40 is attached to wheel 14 and is concentric with wheel well 18. Individual heat shield sections 42 may be secured in place between wheel well 18 and rotors 32 by respective heat shield carriers 44 fixed to wheel well 18. Air gap 46 is defined annularly between heat shield segments 42 and wheel well 18.

Torque bars 24 and heat shield carriers 44 can be secured to wheel 14 using bolts or other fasteners. Torque bar bolts 48 can extend through a hole formed in a flange or other mounting surface on wheel 14. Each torque bar 24 can optionally include at least one torque bar pin 50 at an end opposite torque bar bolts 48, such that torque bar pin 50 can be received through wheel web hole 52 in web 20. Heat shield sections 42 and respective heat shield carriers 44 can then be fastened to wheel well 18 by heat shield fasteners 53.

Under the operating conditions (e.g., high temperature) of aircraft wheel braking assembly 10, carbon-carbon composites may be prone to material loss from oxidation of the carbon. For example, various carbon-carbon composite components of aircraft wheel braking assembly 10 may experience both catalytic oxidation and inherent thermal oxidation caused by heating the composite during operation. In various embodiments, composite rotors 32 and stators 34 may be heated to sufficiently high temperatures that may oxidize the carbon surfaces exposed to air. At elevated temperatures, infiltration of air and contaminants may cause internal oxidation and weakening, especially in and around brake rotor lugs 54 or stator slots 56 securing the friction disks to the respective torque bar 24 and torque take-out assembly 22. Because carbon-carbon composite components of aircraft wheel braking assembly 10 may retain heat for a substantial time period after slowing the aircraft, oxygen from the ambient atmosphere may react with the carbon matrix and/or carbon fibers to accelerate material loss. Further, damage to brake components may be caused by the oxidation enlargement of cracks around fibers or enlargement of cracks in a reaction-formed porous barrier coating (e.g., a silicon-based barrier coating) applied to the carbon-carbon composite.

Elements identified in severely oxidized regions of carbon-carbon composite brake components include potassium (K) and sodium (Na). These alkali contaminants may come into contact with aircraft brakes as part of cleaning or de-icing materials. Other sources include salt deposits left from seawater or sea spray. These and other contaminants (e.g. Ca, Fe, etc.) can penetrate and leave deposits in pores of carbon-carbon composite aircraft brakes, including the substrate and any reaction-formed porous barrier coating. When such contamination occurs, the rate of carbon loss by oxidation can be increased by one to two orders of magnitude.

In various embodiments, components of aircraft wheel braking assembly 10 may reach operating temperatures in the range from about 100° C. (212° F.) up to about 900° C. (1652° F.). However, it will be recognized that the oxidation protection systems and methods of the present disclosure may be readily adapted to many parts in this and other braking assemblies, as well as to other carbon-carbon composite structures susceptible to oxidation losses from infiltration of atmospheric oxygen and/or catalytic contaminants. An oxidation protection system may comprise various layers being applied to a composite structure, such as layers formed by a pretreating composition, a first slurry to form a base layer, a second slurry to form a sealing layer, and/or various other layers.

In various embodiments, a method for limiting an oxidation reaction in a composite structure may comprise forming a first slurry by combining a first pre-slurry composition comprising a first phosphate glass composition in the form of a glass frit, powder, or other suitable pulverized form, with a first carrier fluid (such as, for example, water), applying the first slurry to a composite structure, and heating the composite structure to a temperature sufficient to dry the carrier fluid and form an oxidation protection coating on the composite structure, which in various embodiments may be referred to a base layer. The first pre-slurry composition of the first slurry may comprise additives, such as, for example, ammonium hydroxide, ammonium dihydrogen phosphate, nanoplatelets (such as graphene-based nanoplatelets), among others, to improve hydrolytic stability and/or to increase the composite structure's resistance to oxidation, thereby tending to reduce mass loss of composite structure. In various embodiments, a slurry comprising acid aluminum phosphates having an aluminum (Al) to phosphoric acid (H₃PO₄) molar ratio of 1 to 3 or less, such as an Al:H₃PO₄ molar ratio of between 1 to 2 and 1 to 3, tends to provide increased hydrolytic stability without substantially increasing composite structure mass loss. In various embodiments, a slurry comprising acid aluminum phosphates having an Al:H₃PO₄ molar ratio between 1:2 to 1:3 produces an increase in hydrolytic protection and an unexpected reduction in composite structure mass loss.

With initial reference to FIG. 2A, a method 200 for coating a composite structure in accordance with various embodiments is illustrated. Method 200 may, for example, comprise applying an oxidation protection system to non-wearing surfaces of carbon-carbon composite brake components. In various embodiments, method 200 may be used on the back face of pressure plate 36 and/or end plate 38, an inner diameter (ID) surface of stators 34 including slots 56, as well as outer diameter (OD) surfaces of rotors 32 including lugs 54. The oxidation inhibiting composition of method 200 may be applied to preselected regions of a carbon-carbon composite structure that may be otherwise susceptible to oxidation. For example, aircraft brake disks may have the oxidation inhibiting composition applied on or proximate stator slots 56 and/or rotor lugs 54.

In various embodiments, method 200 may comprise forming a first slurry (step 210) by combining a first pre-slurry composition, comprising a first phosphate glass composition in the form of a glass frit, powder, or other suitable pulverized and/or ground form, with a first carrier fluid (such as, for example, water). In various embodiments, the first slurry may comprise an acid aluminum phosphate wherein the molar ratio of Al:H₃PO₄ may be between 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5 to 1:3, between 1:2.7 to 1:3 or between 1:2.9 to 1:3. The first pre-slurry composition of the first slurry may further comprise a boron nitride additive. For example, a boron nitride (such as hexagonal boron nitride) may be added to the first phosphate glass composition such that the resulting first pre-slurry composition comprises between about 10 weight percent and about 30 weight percent of boron nitride, wherein the term “about” in this context only means plus or minus 2 weight percent. Further, the pre-slurry composition may comprise between about 15 weight percent and 25 weight percent of boron nitride, wherein the term “about” in this context only means plus or minus 2 weight percent. Boron nitride may be prepared for addition to the first phosphate glass composition by, for example, ultrasonically exfoliating boron nitride in dimethylformamide (DMF), a solution of DMF and water, or 2-propanol solution. In various embodiments, the boron nitride additive may comprise a boron nitride that has been prepared for addition to the first phosphate glass composition by crushing or milling (e.g., ball milling) the boron nitride. The resulting boron nitride may be combined with the first phosphate glass composition glass frit.

The first phosphate glass composition may comprise one or more alkali metal glass modifiers, one or more glass network modifiers and/or one or more additional glass formers. In various embodiments, boron oxide or a precursor may optionally be combined with the P₂O₅ mixture to form a borophosphate glass, which has improved self-healing properties at the operating temperatures typically seen in aircraft braking assemblies. In various embodiments, the phosphate glass and/or borophosphate glass may be characterized by the absence of an oxide of silicon. Further, the ratio of P₂O₅ to metal oxide in the fused glass may be in the range from about 0.25 to about 5 by weight.

Potential alkali metal glass modifiers may be selected from oxides of lithium, sodium, potassium, rubidium, cesium, and mixtures thereof. In various embodiments, the glass modifier may be an oxide of lithium, sodium, potassium, or mixtures thereof. These or other glass modifiers may function as fluxing agents. Additional glass formers can include oxides of boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof.

Suitable glass network modifiers include oxides of vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof.

The first phosphate glass composition may be prepared by combining the above ingredients and heating them to a fusion temperature. In various embodiments, depending on the particular combination of elements, the fusion temperature may be in the range from about 700° C. (1292° F.) to about 1500° C. (2732° F.). The resultant melt may then be cooled and pulverized and/or ground to form a glass frit or powder. In various embodiments, the first phosphate glass composition may be annealed to a rigid, friable state prior to being pulverized. Glass transition temperature (T_(g)), glass softening temperature (T_(s)) and glass melting temperature (T_(m)) may be increased by increasing refinement time and/or temperature. Before fusion, the first phosphate glass composition comprises from about 20 mol % to about 80 mol % of P₂O₅. In various embodiments, the first phosphate glass composition comprises from about 30 mol % to about 70 mol % P₂O₅, or precursor thereof. In various embodiments, the first phosphate glass composition comprises from about 40 to about 60 mol % of P₂O₅.

The first phosphate glass composition may comprise from about 5 mol % to about 50 mol % of the alkali metal oxide. In various embodiments, the first phosphate glass composition comprises from about 10 mol % to about 40 mol % of the alkali metal oxide. Further, the first phosphate glass composition comprises from about 15 to about 30 mol % of the alkali metal oxide or one or more precursors thereof. In various embodiments, the first phosphate glass composition may comprise from about 0.5 mol % to about 50 mol % of one or more of the above-indicated glass formers. The first phosphate glass composition may comprise about 5 to about 20 mol % of one or more of the above-indicated glass formers. As used herein, mol % is defined as the number of moles of a constituent per the total moles of the solution.

In various embodiments, the first phosphate glass composition can comprise from about 0.5 mol % to about 40 mol % of one or more of the above-indicated glass network modifiers. The first phosphate glass composition may comprise from about 2.0 mol % to about 25 mol % of one or more of the above-indicated glass network modifiers.

In various embodiments, the first phosphate glass composition, excluding the low CTE material, may represented by the formula: a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z)  [1]

In Formula 1, A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof; G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof. A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is a number in the range from 1 to about 5; b is a number in the range from 0 to about 10; c is a number in the range from 0 to about 30; x is a number in the range from about 0.050 to about 0.500; y₁ is a number in the range from about 0.100 to about 0.950; y₂ is a number in the range from 0 to about 0.20; and z is a number in the range from about 0.01 to about 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂). The first phosphate glass composition may be formulated to balance the reactivity, durability and flow of the resulting glass barrier layer for optimal performance.

In various embodiments, first phosphate glass composition in glass frit form may be combined with additional components to form the first pre-slurry composition. For example, crushed first phosphate glass composition in glass frit form may be combined with ammonium hydroxide, ammonium dihydrogen phosphate, nanoplatelets (such as graphene-based nanoplatelets), among other materials and/or substances. For example, graphene nanoplatelets could be added to the first phosphate glass composition in glass frit form. In various embodiments, the additional components may be combined and preprocessed before combining them with first phosphate glass composition in glass frit form. Other suitable additional components include, for example, surfactants such as, for example, an ethoxylated low-foam wetting agent and flow modifiers, such as, for example, polyvinyl alcohol, polyacrylate, or similar polymers. In various embodiments, other suitable additional components may include additives to enhance impact resistance and/or to toughen the barrier coating, such as, for example, at least one of whiskers, nanofibers or nanotubes consisting of nitrides, carbides, carbon, graphite, quartz, silicates, aluminosilicates, phosphates, and the like. In various embodiments, additives to enhance impact resistance and/or to toughen the barrier coating may include silicon carbide whiskers, carbon nanofibers, boron nitride nanotubes and similar materials known to those skilled in the art.

The components in the first pre-slurry composition may have coefficients of thermal expansion (“CTE”) that are significantly higher than the coefficient of thermal expansion of carbon composite material in a composite structure, to which the first slurry may be applied. The CTE of carbon composite materials may range from below zero to about 3.6×10⁻⁶° C.⁻¹. The CTE of phosphate glass, on the other hand, is higher than that of carbon composite materials, ranging from about 12×10⁻⁶° C.⁻¹ to about 14×10⁻⁶° C.⁻¹, wherein the term “about” as used in this context only refers to plus or minus 2×10⁻⁶° C.⁻¹. Therefore, during operation, the thermal expansion of phosphate glass in the first pre-slurry composition of the first slurry may cause cracks in the carbon material in the composite structure, because the carbon material expands at a slower rate than the phosphate glass, and other materials in the first slurry, when exposed to heat.

In various embodiments, materials with low CTEs may be added to the first pre-slurry composition. Materials with low CTEs may be materials having CTEs of 10×10⁻⁶° C.⁻¹ or less, or further, materials having CTEs of 3×10⁻⁶° C.⁻¹ or less. For example, a low CTE material added to the first pre-slurry composition may be beta-spodumene (Li₂O*Al₂O₃*2SiO₂) with a CTE of 0.9×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), beta-eucryptite (Li₂O*Al₂O₃*2SiO₂) with a CTE of −6.2×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), cordierite (Mg₂Al₄Si₅O₁₈) with a CTE of 1.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 800° C. (1472° F.), faujasite ((Na₂,Ca,Mg)_(3.5)[Al₇Si₁₇O₄₈].32(H₂O)) with a CTE of −3.24×10⁻⁶° C.⁻¹, NZP materials such as SrZr₄P₆O₂₄ and CaZr₄P₆O₂₄ with CTEs of 0.6×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.) and NaZr₂P₃O₁₂ with a CTE of −0.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), Al₂Mo₃O₁₂ with a CTE of 2.4×10⁻⁶° C.⁻¹, Al₂W₃O₁₂ with a CTE of 2.2×10⁻⁶° C.⁻¹, MoZr₂P₂O₁₂, WZr₂P₂O₁₂, Zr₂P₂O₁₂, niobium pentoxide (Nb₂O₅) with a CTE of 1.0×10⁶° C. at 25° C. (77° F.) to 1000° C. (1832° F.), aluminum titanate (Al₂TiO₅) with a CTE of 1.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 800° C. (1472° F.), Zr₂P₂O₉ with a CTE of 0.4×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 600° C. (1112° F.), Beryl (Be₃Al₂Si₆O₁₈) with a CTE of 2.0×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), silicon dioxide glass (SiO₂) with a CTE of 0.5×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 1000° C. (1832° F.), SiO₂—TiO₂ glass with a CTE of −0.03×10⁻⁶° C.⁻¹ to 0.05×10⁻⁶° C.⁻¹ at 25° C. (77° F.) to 800° C. (1472° F.), Cu₂O−Al₂O₃—SiO₂ glasses with a CTE of 0.5×10⁻⁶° C.¹ at 25° C. (77° F.) to 500° C. (932° F.), ZERODUR (a material comprising, by weight, 57.2% SiO₂, 25.3% Al₂O₃, 6.5% P₂O₅, 3.4% Li₂O, 2.3% TiO₂, 1.8% ZrO₂, 1.4% ZnO, 1.0% MgO, 0.5% As₂O₃, 0.4% K₂O, and 0.2% Na₂O) with a CTE of 0.12×10⁻⁶° C.⁻¹ at 20° C. (68° F.) to 600° C. (1112° F.), and/or lead magnesium niobate (Pb₃MgNb₂O₉) with a CTE of 1.0×10⁻⁶° C.⁻¹ at −100° C. (−148° F.) to 100° C. (212° F.). In various embodiments, the first slurry may comprise an amount between 0.5% by weight and 50% by weight of the low CTE material. In various embodiments, the first slurry may comprise an amount between 0.5% by weight and 40% by weight of the low CTE material. In various embodiments, the first slurry may comprise an amount between 5% by weight and 30% by weight of the low CTE material. In various embodiments, the first pre-slurry composition may comprise between 0.5% by weight and 95% by weight of the low CTE material, wherein the first pre-slurry composition comprises all the components of the first slurry except the first carrier fluid. In various embodiments, the first pre-slurry composition may comprise between 1% by weight and 90% by weight of the low CTE material. In various embodiments, the first pre-slurry composition may comprise between 5% by weight and 80% by weight of the low CTE material. In various embodiments, the first pre-slurry composition may comprise between 10% by weight and 30% by weight of the low CTE material.

By adding a low CTE material, such as those listed herein, to the first pre-slurry composition, the difference between the CTE of the carbon material in the composite structure and the average CTE of the first slurry, which will form the base layer after heating (step 230, described below), may be decreased. Therefore, by tailoring the thermal expansion properties of the base layer through the addition of one or more low CTE materials, in response to being exposed to heat, the base layer and the carbon material in the composite structure may expand at more similar rates than a base layer without a low CTE material. More similar CTEs between the carbon material in the composite structure and the base layer formed by the first slurry may help maintain the structural integrity of the composite structure by alleviating cracking in the carbon material and/or cracking in the base layer in response to being heated.

In various embodiments, method 200 further comprises applying the first slurry to a composite structure (step 220). Applying the first slurry may comprise, for example, spraying or brushing the first slurry of the first phosphate glass composition on to an outer surface of the composite structure. Any suitable manner of applying the base layer to the composite structure is within the scope of the present disclosure. As referenced herein, the composite structure may refer to a carbon-carbon composite structure.

In various embodiments, method 200 further comprises a step 230 of heating the composite structure to a temperature sufficient to form a base layer of phosphate glass, which may be referred to as a base layer temperature. The composite structure may be heated (e.g., dried or baked) at a temperature in the range from about 200° C. (292° F.) to about 1000° C. (1832° F.). In various embodiments, the composite structure is heated to a temperature in a range from about 600° C. (1112° F.) to about 1000° C. (1832° F.), or between about 200° C. (292° F.) to about 900° C. (1652° F.), or further, between about 400° C. (752° F.) to about 850° C. (1562° F.). Step 230 may, for example, comprise heating the composite structure for a period between about 0.5 hour and about 8 hours, wherein the term “about” in this context only means plus or minus 0.25 hours. The base layer may also be referred to as a coating.

In various embodiments, the composite structure may be heated to a lower temperature first (for example, about 30° C. (86° F.) to about 400° C. (752° F.)) to bake or dry the base layer at a controlled depth. A second, higher temperature (for example, about 300° C. (572° F.) to about 1000° C. (1832° F.)) may then be used to form a deposit from the base layer within the pores of the composite structure. The duration of each heating step can be determined as a fraction of the overall heating time and can range from about 10% to about 50%, wherein the term “about” in this context only means plus or minus 5%. In various embodiments, the duration of the lower temperature heating step(s) can range from about 20% to about 40% of the overall heating time, wherein the term “about” in this context only means plus or minus 5%. The lower temperature step(s) may occupy a larger fraction of the overall heating time, for example, to provide relatively slow heating up to and through the first lower temperature. The exact heating profile will depend on a combination of the first lower temperature and desired depth of the drying portion.

Step 230 may be performed in an inert environment, such as under a blanket of inert gas or less reactive gas (e.g., nitrogen (N₂), argon, other noble gases and the like). For example, a composite structure may be pretreated or warmed prior to application of the base layer to aid in the penetration of the base layer. Step 230 may be for a period of about 2 hours at a temperature of about 600° C. (1112° F.) to about 800° C. (1472° F.), wherein the term “about” in this context only means plus or minus 10° C. The composite structure and base layer may then be dried or baked in a non-oxidizing, inert or less reactive atmosphere, e.g., noble gasses and/or nitrogen (N₂), to optimize the retention of the first pre-slurry composition of the base layer in the pores of the composite structure. This retention may, for example, be improved by heating the composite structure to about 200° C. (392° F.) and maintaining the temperature for about 1 hour before heating the carbon-carbon composite to a temperature in the range described above. The temperature rise may be controlled at a rate that removes water without boiling, and provides temperature uniformity throughout the composite structure.

In various embodiments and with reference now to FIG. 2B, method 300, which comprises steps also found in method 200, may further comprise applying at least one of a pretreating composition or a barrier coating (step 215) prior to applying the first slurry. Step 215 may, for example, comprise applying a first pretreating composition to an outer surface of a composite structure, such as a component of aircraft wheel braking assembly 10. In various embodiments, the first pretreating composition comprises an aluminum oxide in water. For example, the aluminum oxide may comprise an additive, such as a nanoparticle dispersion of aluminum oxide (for example, NanoBYK-3600®, sold by BYK Additives & Instruments). The first pretreating composition may further comprise a surfactant or a wetting agent. The composite structure may be porous, allowing the pretreating composition to penetrate at least a portion of the pores of the composite structure.

In various embodiments, after applying the first pretreating composition, the component may be heated to remove water and fix the aluminum oxide in place. For example, the component may be heated between about 100° C. (212° F.) and 200° C. (392° F.), and further, between 100° C. (212° F.) and 150° C. (302° F.).

Step 215 may further comprise applying a second pretreating composition. In various embodiments, the second pretreating composition comprises a phosphoric acid and an aluminum phosphate, aluminum hydroxide, and/or aluminum oxide. The second pretreating composition may further comprise, for example, a second metal salt such as a magnesium salt. In various embodiments, the aluminum to phosphorus molar ratio of the aluminum phosphate is 1 to 3 or less. Further, the second pretreating composition may also comprise a surfactant or a wetting agent. In various embodiments, the second pretreating composition is applied to the composite structure atop the first pretreating composition. The composite structure may then, for example, be heated. In various embodiments, the composite structure may be heated between about 600° C. (1112° F.) and about 800° C. (1472° F.), and further, between about 650° C. (1202° F.) and 750° C. (1382° F.).

In various embodiments, step 215 may include applying a single pretreating composition to an outer surface of a composite structure rather than a first pretreating composition and a second pretreating composition, as discussed herein. The single pretreating composition may comprise acid aluminum phosphate, which may in various embodiments, have a molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3. In various embodiments, the single pretreating composition may comprise acid aluminum phosphate with a molar ratio of aluminum to phosphate between 1 to 2 and 1 to 2.7. In various embodiments, the single pretreating composition may comprise acid aluminum phosphate with a molar ratio of aluminum to phosphate of 1 to 2.5. In various embodiments, the single pretreating composition may be substantially free of aluminum oxide. As used herein “substantially free” means comprising less than 0.01% by weight of a substance. In various embodiments, following the application of the single pretreating composition, the composite structure may be heated to a temperature sufficient to form an aluminum phosphate polymer layer on the composite structure comprising aluminum orthophosphate (AlPO₄) and/or aluminum metaphosphate (Al(PO₃)₃). Such a temperature may be referred to as a pretreating composition temperature. In various embodiments, the composite structure with the single pretreating composition may be heated between about 290° C. (554° F.) and about 900° C. (1652° F.), or between about 490° C. (914° F.) and about 625° C. (1157° F.), or between about 600° C. (1112° F.) and about 800° C. (1472° F.), or between about 650° C. (1202° F.) and about 750° C. (1382° F.). The single pretreating composition may further comprise a surfactant or a wetting agent. The composite structure may be porous, allowing the single pretreating composition to penetrate at least a portion of the pores of the composite structure.

A single step of applying and/or heating the single pretreating composition should cost less and take less time than applying a first pretreating composition and, optionally, second pretreating composition, which would involve aluminum oxide, two application steps, and two heating steps.

Step 215 may further comprise applying a barrier coating to an outer surface of a composite structure, such as a component of aircraft wheel braking assembly 10. In various embodiments, the barrier coating composition may comprise carbides or nitrides, including at least one of a boron nitride, silicon carbide, titanium carbide, boron carbide, silicon oxycarbide, and silicon nitride. In various embodiments, the barrier coating may be formed by treating the composite structure with molten silicon. The molten silicon is reactive and may form a silicon carbide barrier on the composite structure. Step 215 may comprise, for example, application of the barrier coating by spraying, chemical vapor deposition (CVD), molten application, or brushing the barrier coating composition on to the outer surface of the carbon-carbon composite structure. Any suitable manner of applying the base layer to composite structure is within the scope of the present disclosure.

In various embodiments and with reference now to FIG. 2C, method 400 may further comprise a step 240, similar to step 210, of forming a second slurry by combining a second pre-slurry composition, which may comprise a second phosphate glass composition in glass frit or powder form, with a second carrier fluid (such as, for example, water). In various embodiments, the second slurry may comprise an acid aluminum phosphate wherein the molar ratio of aluminum (Al) to phosphoric acid (H₃PO₄) may be between 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5 to 1:3, between 1:2.7 to 1:3 or between 1:2.9 to 1:3. In various embodiments, the second slurry may comprise a second pre-slurry composition substantially free of phosphate glass, comprising acid aluminum phosphate and orthophosphoric acid with an aluminum to phosphate molar ratio of 1:2 to 1:5. As used herein “substantially free” means comprising less than 0.01% by weight of a substance. Further, step 240 may comprise spraying or brushing the second slurry of the second phosphate glass composition on to an outer surface of the base layer. In various embodiments, the second slurry may be applied directly to the layer formed by the pretreating composition(s) and/or barrier layer of step 215, such as an aluminum phosphate polymer layer formed from the single pretreating composition discussed herein, such that the oxidation protection system does not include a base layer formed by heating a first slurry. Any suitable manner of applying the sealing layer is within the scope of the present disclosure.

In various embodiments, the second slurry may be substantially free of boron nitride. In this case, “substantially free” means less than 0.01 percent by weight. For example, the second pre-slurry composition may comprise any of the components of the pre-slurry compositions and/or glass compositions described in connection with the first pre-slurry composition and/or first phosphate glass composition, without the addition of a boron nitride additive. In various embodiments, the second pre-slurry mixture may comprise the same pre-slurry composition and/or phosphate glass composition used to prepare the first pre-slurry composition and/or the first phosphate glass composition. In various embodiments, the second pre-slurry composition may comprise a different pre-slurry composition and/or phosphate glass composition than the first pre-slurry composition and/or first phosphate glass composition.

In various embodiments, the first slurry and/or the second slurry may comprise an additional metal salt. The cation of the additional metal salt may be multivalent. The metal may be an alkaline earth metal or a transition metal. In various embodiments, the metal may be an alkali metal. The multivalent cation may be derived from a non-metallic element and/or a metalloid element such as boron. The metal of the additional metal salt may be an alkaline earth metal such as calcium, magnesium, strontium, barium, or a mixture of two or more thereof. The metal for the additional metal salt may be iron, manganese, tin, zinc, or a mixture of two or more thereof. The anion for the additional metal salt may be an inorganic anion such as a phosphate, halide, sulfate or nitrate, or an organic anion such as acetate. In various embodiments, the additional metal salt may be an alkaline earth metal salt such as an alkaline earth metal phosphate. In various embodiments, the additional metal salt may be a magnesium salt such as magnesium phosphate. In various embodiments, the additional metal salt may be an alkaline earth metal nitrate, an alkaline earth metal halide, an alkaline earth metal sulfate, an alkaline earth metal acetate, or a mixture of two or more thereof. In various embodiments, the additional metal salt may be magnesium nitrate, magnesium halide, magnesium sulfate, or a mixture of two or more thereof. In various embodiments, the additional metal salt may comprise: (i) magnesium phosphate; and (ii) a magnesium nitrate, magnesium halide, magnesium sulfate, or a mixture of two or more thereof.

The additional metal salt may be selected with reference to its compatibility with other ingredients in the first slurry and/or the second slurry. Compatibility may include metal phosphates that do not precipitate, flocculate, agglomerate, react to form undesirable species, or otherwise become segregated from the first slurry prior to application of the first slurry and/or the second slurry to the carbon-carbon composite. The phosphates may be monobasic (H₂PO₄ ⁻), dibasic (HPO₄ ⁻²), or tribasic (PO₄ ⁻³). The phosphates may be hydrated. Examples of alkaline earth metal phosphates that may be used include calcium hydrogen phosphate (calcium phosphate, dibasic), calcium phosphate tribasic octahydrate, magnesium hydrogen phosphate (magnesium phosphate, dibasic), magnesium phosphate tribasic octahydrate, strontium hydrogen phosphate (strontium phosphate, dibasic), strontium phosphate tribasic octahydrate and barium phosphate.

In various embodiments, a chemical equivalent of the additional metal salt may be used as the additional metal salt. Chemical equivalents include compounds that yield an equivalent (in this instance, an equivalent of the additional metal salt) in response to an outside stimulus such as, temperature, hydration, or dehydration. For example, equivalents of alkaline earth metal phosphates may include alkaline earth metal pyrophosphates, hypophosphates, hypophosphites and orthophosphites. Equivalent compounds include magnesium and barium pyrophosphate, magnesium and barium orthophosphate, magnesium and barium hypophosphate, magnesium and barium hypophosphite, and magnesium and barium orthophosphite.

While not wishing to be bound by theory, it is believed that the addition of multivalent cations, such as alkaline earth metals, transition metals, nonmetallic elements, and/or metalloids such as boron, to the first slurry and/or the second slurry enhances the hydrolytic stability of the metal-phosphate network. In general, the hydrolytic stability of the metal-phosphate network increases as the metal content increases, however a change from one metallic element to another may influence oxidation inhibition to a greater extent than a variation in the metal-phosphate ratio. The solubility of the phosphate compounds may be influenced by the nature of the cation associated with the phosphate anion. For example, phosphates incorporating monovalent cations such as sodium orthophosphate or phosphoric acid (hydrogen cations) are very soluble in water while (tri)barium orthophosphate is insoluble. Phosphoric acids can be condensed to form networks but such compounds tend to remain hydrolytically unstable. Generally, it is believed that the multivalent cations link phosphate anions creating a phosphate network with reduced solubility. Another factor that may influence hydrolytic stability is the presence of —P—O—H groups in the condensed phosphate product formed from the first slurry and/or the second slurry during thermal treatment. The first slurry and/or the second slurry may be formulated to minimize concentration of these species and any subsequent hydrolytic instability. Whereas increasing the metal content may enhance the hydrolytic stability of the first slurry and/or the second slurry, it may be desirable to balance composition stability and effectiveness as an oxidation inhibitor.

In various embodiments, the additional metal salt may be present in the first slurry and/or the second slurry at a concentration in the range from about 0.5 weight percent to about 30 weight percent, and in various embodiments from about 0.5 weight percent to about 25 weight percent, and in various embodiments from about 5 weight percent to about 20 weight percent. In various embodiments, a combination of two or more additional metal salts may be present at a concentration in the range from about 10 weight percent to about 30 weight percent, and in various embodiments from about 12 weight percent to about 20 weight percent.

Method 400 may further comprise a step 250 of heating the composite structure to a temperature sufficient to form a sealing layer, which may comprise phosphate glass. In various embodiments, the sealing layer may be formed over the layer formed by the pretreating composition and/or barrier layer of step 215, such as an aluminum phosphate polymer layer formed from the single pretreating composition discussed herein, such that the oxidation protection system does not include a base layer formed by heating a first slurry. In various embodiments, the sealing layer may be formed over the base layer. Similar to step 230, the composite structure may be heated to a temperature sufficient to adhere the sealing layer to the base layer or the layer formed by the pretreating composition and/or barrier layer (e.g., the aluminum phosphate polymer layer). Such a temperature may be referred to as a sealing layer temperature. For example, step 250 may comprise drying or baking the carbon-carbon composite structure at a temperature in the range from about 200° C. (392° F.) to about 1000° C. (1832° F.). In various embodiments, the composite structure is heated to a temperature in a range from about 600° C. (1112° F.) to about 1000° C. (1832° F.), or between about 200° C. (392° F.) to about 900° C. (1652° F.), or further, between about 400° C. (752° F.) to about 850° C. (1562° F.), wherein in this context only, the term “about” means plus or minus 10° C. Further, step 250 may, for example, comprise heating the composite structure for a period between about 0.5 hour and about 8 hours, where the term “about” in this context only means plus or minus 0.25 hours.

In various embodiments, step 250 may comprise heating the composite structure to a first, lower temperature (for example, about 30° C. (86° F.) to about 300° C. (572° F.)) followed by heating at a second, higher temperature (for example, about 300° C. (572° F.) to about 1000° C. (1832° F.)). Further, step 250 may be performed in an inert environment, such as under a blanket of inert or less reactive gas (e.g., nitrogen, argon, other noble gases, and the like).

TABLE 1 illustrates a variety of phosphate glass compositions in accordance with various embodiments.

TABLE 1 Phosphate Glass Composition A B C D E F Wt % Glass 75.01 75.01 76.71 69.74 80.20 73.22 Wt % Boron Nitride 0 0 21.07 28.09 17.53 24.54 Wt % o-AlPO4 0 2.27 0 0 0 0

Phosphate glass compositions A and B comprise boron nitride-free phosphate glasses. For example, glasses A and B may be suitable sealing layers, such as the sealing layer applied in step 240 of method 400. Phosphate glass compositions C through F comprise boron nitride-containing phosphate glass. For example, glass compositions C through F may illustrate suitable base layers, such as base layers formed by heating a first slurry in step 230 of methods 200, 300, and 400. As illustrated, the boron nitride content of glass compositions C through F varies between about 17.53 and 28.09 weight percent boron nitride. However, any suitable boron nitride-containing phosphate glass (as described above) is in accordance with the present disclosure.

With reference to FIG. 3 and TABLE 2 (below), experimental data obtained from testing various glass compositions in accordance with various embodiments is illustrated.

TABLE 2 Base Layer (none) E D C F Sealing Layer Oxidation Exposure A A A A A Temp Time (Hours) Percentage Weight Loss 675 Degrees C. 0 0.00 0.00 0.00 0.00 0.00 4 0.90 0.32 0.41 0.44 0.18 8 1.98 0.70 1.01 1.07 0.39 12 3.13 1.19 1.76 1.95 0.64 16 4.41 1.80 2.56 2.98 0.95 20 5.94 2.52 3.56 4.65 1.31 24 7.48 3.41 4.73 6.29 1.76 760 Degrees C. 26 10.63 4.31 6.43 8.47 2.11 28 14.28 5.40 8.37 11.13 2.54 870 Degrees C. 30 24.43 7.18 11.96 15.95 3.50 32 36.45 12.76 17.27 23.43 6.89

The base layer and sealing layers shown in TABLE 2 are the same as shown in TABLE 1, with like labeling A through F. As illustrated in TABLE 2, a base layer of phosphate glass formed of boron nitride-comprising glass compositions C through F are applied as a slurry (i.e., a first slurry of a first phosphate glass composition, such as that formed in step 210 in FIGS. 2A-2C) to a composite structure. A sealing layer formed of glass composition A is applied over the base layer (i.e., a second slurry of a second phosphate glass composition, such as that formed in step 240 in FIG. 2C). As shown, the composite structure having a base layer exhibited a lower weight loss to oxidation at temperatures at and above 675° C. (1250° F.) than composite structures having sealing layer A by itself. FIG. 3 illustrates the data from TABLE 2, with percent weight loss on the y axis and exposure time on the x axis.

With reference to FIG. 4, a number of combinations are illustrated, including various combinations of pre-treatment (such as, for example, as may be performed by pretreatment step 215 with a first pretreating composition and/or second pretreating composition), base layers, and sealing layers. The base layer and sealing layers are shown in FIG. 4 with the same labeling as shown in TABLE 1, A through F. For example, FIG. 4 illustrates that the performance of a composition comprising a base layer of glass composition F and a sealing layer B provides improved oxidation protection over a pretreated composite structure having only sealing layer B. Other combinations include a base layer of glass composition F and a sealing layer A (without pre-treatment), a pretreatment with sealing layer A only, a base layer of F with a sealing layer of A (with pre-treatment).

TABLE 3 illustrates a variety of slurries comprising phosphate glass compositions and prepared in accordance with various embodiments.

TABLE 3 Example >> A B1 B2 C D E F G H h-Boron nitride powder A 0 0 0 8.75 8.75 8.25 8.25 8.25 8.25 h-Boron nitride powder B 0 0 0 0 0 0.5 0.5 0.5 0.5 Graphene nanoplatelets 0 0 0 0.15 0.15 0.15 0.15 0.15 0.15 H₂O 52.4 52.4 52.4 50.0 50.0 45.0 45.0 60.0 60.0 Surfynol 465 surfactant 0 0 0 0.2 0.2 0.2 0.2 0.2 0.2 Ammonium dihydrogen phosphate (ADHP) 11.33 11.33 1.0 0 0 0 0 0.5 0 NH₄OH 0 0 0 0.5 0 0.5 0.5 0 0.5 Glass frit 34.0 34.0 34.0 26.5 26.5 26.5 26.5 26.5 26.5 Acid aluminum phosphate (1:3 Al-P) 0 0 0 0 0 10.0 0 0 0 Acid aluminum phosphate (1:2.1 Al-P) 0 0 10.0 0 0 0 0 5.0 0 Acid aluminum phosphate (1:2.5 Al-P) 0 0 0 0 0 0 0 0 5.0 Acid aluminum phosphate (1:2 Al-P) 0 0 0 0 0 0 10.0 0 0 Ammonium dihydrogen phosphate (ADHP) 0 0 0 0 0.5 0 0 0 0 Aluminum orthophosphate (o-AlPO₄) 0 2.27 2.27 0 0 0 0 0 0

TABLE 4 illustrates a variety of acid aluminum phosphate solutions in accordance with various embodiments.

TABLE 4 Acid aluminum phosphate solutions AlPO 1:4 AlPO 1:3.5 AlPO 1:3 AlPO 1:2.5 AlPO 1:2 Component MW Amount (grams) H₂O 18.01 60.10 60.65 61.42 62.50 64.08 H₃PO₄ (85%) 98.00 100.00 100.00 100.00 100.00 100.00 Al(OH)₃ 78.00 19.90 22.74 26.53 31.85 39.80 Al:P (molar ratio) 0.250 0.286 0.333 0.400 0.500

As illustrated in TABLES 3 and 4, oxidation protection system slurries comprising a phosphate glass composition glass frit in a carrier fluid (i.e., water) and various additives including h-boron nitride, graphene nanoplatelets, a surfactant, a flow modifier such as, for example, polyvinyl alcohol, polyacrylate or similar polymer, ammonium dihydrogen phosphate, ammonium hydroxide, and acid aluminum phosphates with Al:H₃PO₄ molar ratios of between 1 to 2 and 1 to 3 were prepared. Such as, for example, slurry example H contained h-boron nitride and an acid aluminum phosphate solution with an aluminum to phosphorus molar ratio of 1:2.5. Phosphate glass compositions in slurries A, B1, and B2 comprise boron nitride-free phosphate glasses. For example, glasses A, B1, and B2 may be suitable sealing layers, such as the sealing layer formed by the application of second slurries in step 240 of method 400. Phosphate glass compositions C through H comprise boron nitride-containing phosphate glass. For example, glass compositions C through H may illustrate suitable base layers, such as base layers formed by the application of first slurries in step 210 of methods 200, 300, and 400.

With combined reference to TABLE 3 and FIG. 5, the first slurries (examples C, D, E, F, and H) were applied to 50 gram carbon-carbon composite structure coupons and cured in inert atmosphere under heat at 899° C. (1650° F.) to form base layers. After cooling, second slurries (examples A, B1 or B2) were applied atop the cured base layer and the coupons were fired again in an inert atmosphere. A control coupon was pretreated with an alumina nanoparticle (i.e., a first pretreating composition as discussed in association with step 215) and given an acid aluminum phosphate layer (i.e., a second pretreating composition as discussed in association with step 215) with a Al:H₃PO₄ molar ratio of about 1 to 3.3, as described in various embodiments, and cured under an inert atmosphere. A control was prepared with a second slurry comprising a phosphate glass composition, water, ammonium dihydrogen phosphate, and aluminum orthophosphate. The second slurry for the control (example B1) was applied atop the cured pretreated control and then fired again under an inert atmosphere forming, a sealing layer. After cooling, the coupons were subjected to isothermal oxidation testing a 760° C. (1400° F.) over a period of hours while monitoring mass loss.

With further reference to TABLE 3 and FIG. 5, the performance of the coatings applied according to various embodiments is illustrated in comparison with a control. The control includes a pretreated composite structure having only sealing layer from slurry B1. Percent weight loss is shown in the y axis and exposure time is shown in the x axis. Against the control, the addition of an acid aluminum phosphate with molar ratio of Al:H₃PO₄ of between 1:2 and 1:3 reduces mass losses due to oxidation by between two times to over ten times (i.e., an order of magnitude). After 24 hours at 760° C. (1400° F.) the control had lost 24.1% of its mass in comparison the best performing test sample which had lost only 0.7% of its mass. The combined effect of adding acid aluminum phosphate into the slurry wherein the molar ratio of Al:H₃PO₄ is between about 1:2 and 1:3 provides an unexpected increase in protection over standalone pretreatment of the carbon-carbon composite structure followed by application of a base layer or a base layer and sealing layer. Without being bound by theory, it is thought that the addition of acid aluminum phosphate wherein the molar ratio of Al:H₃PO₄ is between about 1:2 and 1:3 produces a base layer which is both cohesive and adhesive.

With reference to Table 5 (below) and FIG. 6, experimental data obtained from testing various oxidation protection systems in accordance with various embodiments is illustrated.

TABLE 5 Data Set 610 605 Pretreatment First and Single pretreating Second pretreating composition compositions Base Layer (none) (none) Sealing Layer Oxidation Exposure B1 B1 Temp Time (Hours) Percentage Weight Loss 760 Degrees C. 0 0.00 0.00 4 0.39 0.38 8 1.68 1.56 12 4.41 4.63 16 8.47 9.57 20 13.08 15.94 24 20.90 24.11

Table 5 and FIG. 6 show the performance of pretreatment compositions discussed herein in relation to step 215 in FIGS. 2B and 2C, which may be applied to a composite structure prior to the application of a first slurry (step 210) and/or a second slurry (step 240). Percent weight loss of the carbon structure due to oxidation is shown on the y-axis, and exposure time to a temperature of at 760° C. (1400° F.) in hours is shown on the x-axis.

Data sets 605 and 610 in Table 5 and FIG. 6 show experimental data for composite structures with oxidation protection systems comprising pretreatment compositions and a sealing layer formed by a second slurry B1 (in Table 3). Data set 605 displays experimental data for a composite structure that had been pretreated with a first pretreating composition comprising aluminum oxide and a second pretreating composition comprising acid aluminum phosphate with an aluminum to phosphate molar ratio of 1 to 3.3, as discussed in relation to step 215 in FIGS. 2B and 2C, before slurry B1 was applied to the composite structure. Data set 610 displays experimental data for a composite structure that had been pretreated with a single pretreating composition comprising acid aluminum phosphate with an aluminum to phosphate molar ratio of 1 to 2.5, as discussed in relation to step 215 in FIGS. 2B and 2C, before slurry B1 was applied to the composite structure. After 24 hours at 760° C. (1400° F.) the composite structure reflected by data set 605 had lost 24.11% of its mass, while the composite structure reflected by data set 610 had lost 20.90%. Therefore, the effect of a single pretreating composition, as described herein, being applied to a composite structure provides an unexpected increase in oxidation protection for the composite structure from a pretreatment comprising a first pretreating composition and a second pretreating composition. Additionally, as discussed herein, pretreatment of a composite structure involving a single pretreating composition allows for cost and time savings as opposed to the pretreatment involving the first pretreating composition and/or second pretreating composition.

Table 6 (below) shows experimental data obtained from testing various oxidation protection systems comprising a substantially phosphate glass-free sealing slurry comprising acid aluminum phosphate and orthophosphoric acid, in accordance with various embodiments.

TABLE 6 Base Layer G G Sealing Layer Oxidation Exposure (none) J Temp Time (Hours) Percentage Weight Loss 760 Degrees C. 0 0.00 0.00 4 8.34 0.50 8 23.10 2.15 12 43.35 5.55 16 — 11.03 20 — 17.82 24 — 25.84

As illustrated in TABLE 6, oxidation protection systems with various slurries were tested for effectiveness at preventing oxidation on composite structures. Slurry example G (from Table 3) contained a first pre-slurry composition comprising a first phosphate glass composition and various additives including h-boron nitride, graphene nanoplatelets, a surfactant, ammonium hydroxide, and acid aluminum phosphate with Al:H₃PO₄ a molar ratio of 1 to 2.1. The first pre-slurry composition from slurry example G was combined with a carrier fluid (i.e., water). Slurry G was applied to 50 gram carbon-carbon composite structure coupons and cured in inert atmosphere under heat at 899° C. (1650° F.). After cooling, a second slurry J comprising acid aluminum phosphate and/or orthophosphoric acid with a molar ratio of aluminum to phosphate of between 1 to 2 and 1 to 5 was applied atop the cured base layer and the coupons were fired again in an inert atmosphere. Second slurry J is substantially free of phosphate glass.

With further reference to TABLE 6, the performance of the sealing slurry J, which creates a sealing layer in response to being heated in step 250, applied to a composite structure according to various embodiments is illustrated in comparison with a control. The control includes a pretreated composite structure having only base layer G. Against the control, the addition of the second slurry J reduces mass losses due to oxidation by between five times to over ten times (i.e., an order of magnitude). After 12 hours at 760° C. (1400° F.) the control had lost 43.35% of its mass in comparison to the composite structure with base layer G and sealing layer J, which lost only 5.55% of its mass. The effect of adding a substantially phosphate glass-free second slurry (i.e., slurry J) to the composite structure comprising acid aluminum phosphate and orthophosphoric acid wherein the molar ratio of Al:PO₄ is between about 1:2 and 1:5 provides an unexpected increase in protection over standalone pretreatment of the carbon-carbon composite structure followed by application of only a base layer.

Benefits and other advantages have been described herein with regard to specific embodiments. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical system. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the disclosure. The scope of the disclosure is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, where a phrase similar to “at least one of A, B, or C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C.

Systems, methods and apparatus are provided herein. In the detailed description herein, references to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. After reading the description, it will be apparent to one skilled in the relevant art(s) how to implement the disclosure in alternative embodiments.

Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element is intended to invoke 35 U.S.C. 112(f) unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. 

What is claimed is:
 1. A method for coating a composite structure, comprising: applying a single pretreating composition on a surface of the composite structure, the single pretreating composition comprising an acid aluminum phosphate comprising a molar ratio of aluminum to phosphate between 1 to 2 and 1 to 3; and heating the composite structure to a pretreating composition temperature sufficient to form an aluminum phosphate polymer layer on the composite structure; applying a first slurry to the aluminum phosphate polymer layer and heating the composite structure to a base layer temperature sufficient to form a base layer, wherein the first slurry comprises a first pre-slurry composition and a first carrier fluid, wherein the first pre-slurry composition comprises a first phosphate glass composition; applying a sealing slurry to the base layer, and heating the composite structure to a sealing layer temperature sufficient to form a sealing layer on the base layer, wherein the sealing slurry comprises a sealing pre-slurry composition and a second carrier fluid, wherein the sealing pre-slurry composition comprises a second phosphate glass composition.
 2. The method of claim 1, wherein the first pre-slurry composition comprises a first acid aluminum phosphate having a first molar ratio of aluminum to phosphate of between 1 to 2 and 1 to
 3. 3. The method of claim 1, wherein the molar ratio of aluminum to phosphate in the acid aluminum phosphate of the single pretreating composition is between 1 to 2 and 1 to 2.7.
 4. The method of claim 1, further comprising applying a barrier coating to the composite structure after the applying the single pretreating composition to the composite structure.
 5. The method of claim 4, wherein the barrier coating comprises at least one of a carbide, a nitride, a boron nitride, a silicon carbide, a titanium carbide, a boron carbide, a silicon oxycarbide, a molybdenum disulfide, a tungsten disulfide, or a silicon nitride.
 6. The method of claim 1, wherein at least one of the first phosphate glass composition or the second phosphate glass composition is represented by the formula a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z): A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof; G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is a number in the range from 1 to about 5; b is a number in the range from 0 to about 10; c is a number in the range from 0 to about 30; x is a number in the range from about 0.050 to about 0.500; y₁ is a number in the range from about 0.100 to about 0.950; y₂ is a number in the range from 0 to about 0.20; and z is a number in the range from about 0.01 to about 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂).
 7. The method of claim 1, wherein the first slurry comprises a refractory compound such as a nitride, a boron nitride, a silicon carbide, a titanium carbide, a boron carbide, a silicon oxycarbide, silicon nitride, molybdenum disulfide, or tungsten disulfide.
 8. The method of claim 1, wherein at least one of the first slurry or the sealing slurry comprises at least one of a surfactant, a flow modifier, a polymer, ammonium hydroxide, ammonium dihydrogen phosphate, acid aluminum phosphate, nanoplatelets, or graphene nanoplatelets. 